#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define endl '\n'
const int N = 5e4 + 5;

int L, n, m;
int a[N];
bool check(int d) {
  int cnt = 0;
  int pre = 0;
  rep(i, 1, n) {
    if (a[i] - pre < d)
      cnt++;
    else
      pre = a[i];
  }
  if (pre + d > L) cnt++;
  return cnt <= m;
}
int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> L >> n >> m;
  rep(i, 1, n) cin >> a[i];
  int l = 1, r = 1e9;
  while (l <= r) {
    int mid = (l + r) >> 1;
    if (check(mid))
      l = mid + 1;
    else
      r = mid - 1;
  }
  cout << r;
  return 0;
}